Projection lithography tools are ones of those used as core processing equipment in the field of liquid crystal display (LCD). Through-silicon vias (TSVs) represent the latest technique for interconnection of chips by means of vertical conductive vias fabricated between the chips or wafers. TSVs offer the advantages of small package size, fast signal transmission, low power consumption and etc. Standard wafers subjected to a number of TSV processes often exhibit, at their edge portions, bonding inconsistencies, non-concentricities, wears, scribe lines, sputtered metals or insulating material on the surface or warps. Under such complex conditions, apparatuses for focusing and leveling are often incapable of measurement.
As shown in FIG. 1, a conventional detection device for focusing and leveling comprises a measuring optical path extending on both sides of an optical axis of a projection objective 20. In the measuring optical path, there are disposed an illumination unit, a projection unit, a detection unit and a relay unit in a sequence. Light emanated from a light source 21 of the illumination unit is converged at a light-source lens group 22 and then guided by an optical fiber to the projection unit, so as to provide lighting for the entire detection device. The projection unit is composed of a projection slit 23, a front projection lens group 24, a projection reflector group 25, a rear projection lens group 26 and the like. After propagating through the projection slit and the lens 24, being reflected by the reflector 25 and passing through the lens 26, the light forms a measuring light spot in the current exposure area on the surface of a glass substrate. The detection unit is made up of a front detection lens group 27, a detection reflector group 28, a rear detection lens group 29 and others. The relay unit is constituted of a relay reflector 30, a relay lens group 31, a detector 32, an arithmetic unit 33, a controller 34 and others. Light from the light spot travels through the relay unit and is received by the detector which thereby forms a light intensity signal carrying surface height information of the object under measurement (e.g., a glass substrate). This measurement approach requires the surface of the object under measurement to be flat, but the object under measurement 36, such as a wafer or a glass substrate, carried on the movable carrier stage 35 has trenches 41 (see FIG. 3) due to different processed layers.
As shown in FIGS. 2A and 2B, a solution has been proposed in order to address the issue of trenches 41, in which, an illumination numerical aperture A is designed to be greater than or equal to the product of a projection numerical aperture B multiplied by a detection numerical aperture M, which is, in turn, designed to be greater than or equal to the sum of the detection numerical aperture M and a tilt W of the object under measurement (i.e., A≥B*M≥M+W), in order to reduce shadows in images caused by the surface irregularities of the object under measurement which may lead to measurement errors. In many cases, interlaced patterns and wires are formed on a wafer surface. As a result, deviations in various directions are also generated from reflections. A projection aperture stop 50 may be shaped like a cross so that a distance d is created between the detection and projection light beams, ensuring easy reception of the detection light beam 51 with reduced measurement errors. Although this solution can mitigate the consequences of the object's surface irregularities to a certain extent, for a relative large trench 41 (as shown in FIG. 3), the measurement will be still impossible if the projection light beam is incident on the inner wall surface of the trench.
In the surface of an actual object 36 such as a wafer or the glass substrate, are often formed periodic trenches typically at a pitch on the order of millimeters, e.g., 5 mm, which are unfavorable to the measurement. Since the trenches themselves are sized on the order of hundreds of microns, e.g., 100 μm, it is likely for the measuring light spot to be formed partially or entirely within one of the trenches. This may introduce measurement errors or even make the measurement impossible. As shown in FIG. 3, when the measuring light spot is formed in part or as a whole within one of the trenches, the reflection in the optical path will occurs in an undesirable way in which the reflected light cannot be collected and the measurement of the object is thus affected adversely. When the reflected light is entirely reflected away by the trench, the measurement will be disabled.